Vegetable oils from different origins, e.g. soy, sunflower, rape or maize, are produced by solvent extraction, commonly hexane, followed by refining. The first step in the refining is a so-called degumming, in which the polar lipids are obtained as a precipitate after an addition of approximately 2 wt % water. The polar lipid precipitate is separated from the liquid, dried, and bleach, yielding a product named lecithin. The lecithin product contains 40-60% polar lipids, the remaining fraction being mainly neutral lipids. The lecithin fraction is typically 1 wt % of the starting oil product. Lecithin obtained from soybean oil contains typically 85 wt % phospholipids, 10 wt % glycolipids and 5 wt % other substances (Lecithins: Sources, Manufacture & Use, B. F: Szuhaj, editor, American Oil Chemists' Society, 1989, chap 7).
Polar lipids constitute an important part of the cell wall membrane of the plant material and are known to display a high surface activity, which make them interesting in a multitude of applications.
Oil from oat kernels has attracted an interest as starting material to produce natural emulsifiers in several products, for example in food, cosmetics and pharmaceutical compositions. The oil from oats is obtained through extraction with a solvent, e.g. hexane or aliphatic alcohol's, such as ethanol or iso-propanol. When ethanol is used as a solvent, more polar lipids as well as sugars are extracted from the kernel. The oat oil will then contain 10-20% polar lipids, i.e. much higher levels than oil from traditional oil seeds. In addition, the polar lipids from oats, as well as the polar lipids from other cereals such as wheat and rye, have a different composition compared to the traditional oil seeds. The levels of phospholipids are low and the levels of glycolipids, mainly galactolipids, are high. A recent publication on the composition of the polar lipids in oat can be found in Lipids (2008) 43:533-548.
As the ethanol extracted oat oil is concentrated by evaporation of ethanol, parts of the sugar are dissolved in the oil together with the remaining ethanol and water, the remaining parts of the sugar form a finely dispersed precipitate which is difficult to separate through sedimentation. Particles of starch and protein may also be present in crude extracts.
Because of the high levels of polar lipids and the presence of sugars in this type of oils, traditional refining or “degumming” methods are not feasible. Large amounts of water is required to precipitate all the polar lipids, and instead of forming a separate phase a stable emulsion is produced by the surface active polar lipids. A separation would be very tedious and costly.
Patent SE-B-417 441 describes such a method, in which polar lipids from wheat are obtained from a crude oil by the addition of water. The water can be pure, but it may also contain salt up to 4 wt %. Seven parts of water and three parts of oil formed an emulsion that could be separated into three phases, neutral lipids, a mixed oil phase and an aqueous phase, only by subjecting it to high gravitational forces during a prolonged time. The mixed oil phase contained most of the polar lipids and 50-70% water. It was claimed that this fraction had good bread-making properties.
In U.S. Pat. No. 5,466,782 wheat is extracted using warm ethanol. The plant extract is filtered warm and starch and protein are removed. When the extract is cooled to between −20° C. and +4° C. ceramides are precipitated and a powder with a high content of ceramides can be filtered off. The filtrate is evaporated and an oil with 50% triglycerides, 25% galactolipids and 25% phospholipids is achieved.
In EP1043016 wheat is extracted using warm ethanol. Water and protein are removed from the extract using kieselguhr or celite and filtration, before the extract is concentrated by distillation. The oil is fractionated by addition of an emulsifier and demonized water at 70-90° C. and during 30-60 minutes before the mixture is left for 24 h. A fraction, rich in polar lipids is achieved. This fraction is frozen, freeze dried, grinded and micronized. This extract is claimed to be useful in pharmaceutical, cosmetic and food compositions.
Separations using chromatographic methods are described in several patents, e.g. EP0009842, WO95/20943 and WO9638160. Compounds with very high purity, >95%, can be produced but the process costs are very high using these methods.
Fractionations of oat oil using different solvents have previously been described. In the patent family WO 88/08253 (EP0371601, U.S. Pat. No. 5,026,548), two methods are described to obtain an extract from oats, claimed to have a strong viscosity reducing effect on chocolate. In the first method oat is extracted with a polar solvent, the oil obtained is fractionated by the addition of methanol, resulting in a methanol phase rich in the desired lipids. In the second method oat is first treated with a non-polar solvent, followed by an extraction with polar solvents, namely ethanol or iso-propanol. The latter, polar solvent extract contains the polar lipids. The effect of reduced viscosity is ascribed to the presence of digalactosyl-diglyceride (DGDG) in oats. This compound contains one or more unsaturated hydroxyl-fatty acids that are esterified by other fatty acids of the same type.
The same research group claims later that DGDG with esterified hydroxyl-fatty acids, estolides, only exists in oats (Jee M. H. 1995, “A new emulsifier from oat”, Proc. 21st World Congress ISF, The Hague, paper 135).
In EP-A-0 371 601 a process is described, in which oat oil is fractionated using methanol. After evaporating the methanol phase, an oily fraction remains which display surface-active properties. By adding acetone to this oily fraction a lipid precipitate was obtained. This acetone insoluble material was found to be a good emulsifier, both for oil-in-water and water-in-oil emulsions. This was demonstrated in the production of bread and margarine, as well as formation and stabilisation of aqueous foams.
In WO 97/10050 an additional method for fractionation of oat oil using methanol is described. After evaporation of the methanol extract an oily substance remains. Also in this method the oily remains were further fractionated by addition of acetone. However, in contrast to the method described in EP-A-0 371 601, the surface-active component is obtained from the acetone soluble fraction. The main component in this fraction was claimed to be DGDG. The inventors claim that the acetone insoluble fraction contains mainly hydroxyl-fatty acids. The acetone soluble fraction was found to have a good stabilising effect on water-in-oil emulsions.
In WO 97/11141 a method is described whereby a plant material, for example oat, is extracted with hexane to obtain a crude oil. The hexane-extracted crude oil is further fractionated by the addition of an alcohol and water. In the experiments when ethanol is used the water concentration is below 8 wt %. The yields of recovered polar lipid fractions were 15% and the polar lipid content was approximately 40%, of which 80% were glycolipids and of this 76% DGDG. Several oil-in-water emulsions were successfully prepared using these fractions.
These methods are all rather tedious and time-consuming since they involve several extraction-steps using several different solvents. In addition, the DGDG product is recovered in a low overall yield and in relatively low concentrations of polar lipids. Hence, the methods are poorly feasible from an economical point-of-view.
The traditional extraction methods using hexane as solvent suffers from the difficulty to remove hexane from the solid residues. This residue is mostly used as feed and in this way toxic hexane is brought into our food. Hence, there is a need for methods getting rid of toxic solvent components in the feed.
Thus, an improved method for oil extraction or separation of lipids would be advantageous and in particular a method allowing for increased yield of polar lipids, improved method to handle lipids containing high amounts of polar lipids, increased possibilities to handle sugar, increased possibility to handle insoluble residues and the method should be flexible and cost-effective.